PROJECT SUMMARY/ABSTRACT Acute lymphoblastic leukemia (ALL) is a major cause of illness and death in adults and the most common childhood malignancy. Some types of ALL, including BCR-ABL+ B-lineage ALL, still respond poorly to therapy and require intensive treatment regimens that result in serious toxic side-effects. My laboratory identified that poor prognosis BCR-ABL+ B-ALL leukemic cells are highly dependent on the expression of endogenous anti- apoptotic MCL-1. Now, we wish to harness our knowledge of MCL-1 biology to improve therapeutic responses while reducing detrimental therapy-associated sequelae. We have identified dihydroarteminisin (DHA), a water- soluble metabolite of the anti-malarial arteminisin, which elicits a down-modulation of MCL-1 expression and sensitizes cells to treatment with BH3-mimetics (ABT-199 and ABT-263). Our central hypothesis is that DHA treatment of BCR-ABL+ B-ALL will trigger the specific decrease in MCL-1 expression, thus rendering the cells sensitive to death induced by BH3-mimetics. We propose to determine how MCL-1 expression is effected by DHA, validate its efficacy and specificity in models of leukemia, and test whether the use of DHA can synergize with BH3-mimetic agents leading to more potent therapeutic intervention. We propose to use a series of highly innovative approaches to tackle these longstanding and important questions: Aim 1: Define how DHA triggers cell death in BCR-ABL+ B-ALL. We seek to determine: (1A) Is DHA specific for MCL-1-dependent cells? (1B) Does DHA induced killing depend on NOXA induction? (1C) Does DHA destabilize MCL-1 expression? Aim 2: Validate the potency of DHA at synergizing with BH3-mimetics in vitro. We will investigate whether treatment of mouse and human leukemic cells with DHA can synergize in vitro with BH3-mimetic agents. (2A) Can DHA synergize with BH3-mimetics in wild-type and TKI-resistant mouse leukemia? (2B) Can DHA synergize with BH3-mimetics in human leukemia? Aim 3: Demonstrate the therapeutic potential of DHA at synergizing with anti-BCL-2 therapeutic BH3-mimetics in mouse and xenografts models of leukemia. We will validate the therapeutic potential in a preclinical setting by: (3A) Testing whether DHA can sensitize mouse leukemia to BH3-mimetics in vivo with minimal side-effects. (3B) Validating the efficacy of combining DHA and BH3-mimetics in primary patient derived xenografts models of Ph+ leukemia. My laboratory has made many of the seminal findings defining the role of MCL-1 in promoting survival during hematopoiesis and in leukemia. We are uniquely positioned to successfully perform these studies as we have generated the critical model systems and are collaborating with a world-leader in B-ALL animal modeling, Dr. Charles Mullighan at St. Jude Children's Research Hospital (SJCRH). At the end of this study, we will have illuminated a previously unrecognized combinatorial therapy for treating poor prognosis B-ALL.